1. Field of the Invention
The present invention relates to a method for producing an expanded graphite by using SO.sub.3 gas produced from fuming sulfuric acid or anhydrous sulfuric acid.
2. Description of the Prior Art
Graphites are made of carbon atoms like coke and diamond, and the hexagonal structure of graphite forms a layer structure to give a specific lamellar crystal structure. When graphite is oxidized by using an appropriate oxidant, chemical ions such as SO.sub.3.sup.2- and NO.sub.3 - are introduced between the laminated layers to produce an interlayer compound. When thus obtained graphite is heated, the chemical ions in the interlayer compound become gases and the interlayer compound expands several hundred times to several thousand times of its original size due to the pressure of the generated gases. The c-axis of thus expanded graphite is from 20 to 250 times of c-axis of the common graphite.
Conventionally, expanded graphites are manufactured by oxidizing natural and/or pyrolyzed graphite with a mixture of sulfuric acid and oxidant, such as a mixture of sulfuric acid and nitric acid, sulfuric acid and potassium permanganate, sulfuric acid and hydrogenperoxide, or sulfuric acid and perchloric acid. The oxidized graphite is then washed with water and dried. The dried product is rapidly heated so that thus obtained graphite compound can expand to obtain the resulting expanded graphite.
The above-described conventional method will be described as follows. Japanese Patent Laid-Open No. sho 59-35078 discloses a method for producing an expanded graphite form. The expanded graphite is manufactured by impregnating the graphite with an acid mixture of 50% sulfuric acid and 60% perchloric acid in a ratio of 100:5 by weight and then heating the graphite at 1000.degree. C.
Japanese Patent Laid-Open No. sho 61-72609 discloses a method for producing an expanded graphite by using sodium and tetrahydrofurane. The obtained expanded graphites are commonly used for a material for gasket, a composite material of cement and mortar, an expanded graphite sheet and the like. Additionally, expanded graphites obtained by using acid mixtures containing sulfuric acid as an oxidant and as a main intercalating agent have more specific use as a material for an expanded graphite sheet, an oil adsorbent and an inorganic fiber composite material as disclosed by U.S. Pat. No. 5,149,518. Alternatively, U.S. Pat. No. 5,503,717 discloses a method of producing an expanded graphite by electrochemically immersing ZnCl.sub.2 between the graphite layers in an aqueous solution and then heating thus obtained product.
However, in the above described conventional methods, graphite particles should be impregnated with a large amount of concentrated acid mixture (about 1.5 times of the amount of graphite particles) for a prolonged period of time. Consequently, the consuming amount of the acid mixture is very large while generating a large amount of waste acid. Moreover, since the waste acid contains various compounds of acids and oxidants, the treatment of the waste acid become a complicated task.
Due to these problems, the commercialization of these methods for producing the expanded graphite is difficult to implement. In addition, SO.sub.3 gas and SO.sub.2 gas generated during the producing process of the expanded graphite are environmentally hazardous agents. Further, sulfur remained within the expanded graphite cause corrosion of metals when the expanded graphite is applied to the packing, the gasket sheet and the like. Although these problems are not a factor for an expanded graphite manufactured by utilizing alkaline metal and hydrofuran and the electrochemical method utilizing ZnCl.sub.2, the use of alkaline metal is expensive and a current having a current density of from 2000 to 5000 A/m.sup.2 should be applied, hence increasing the energy consuming cost and so the manufacturing cost.